There have been significant operational, technical and environmental constraints associated with the energy sources used for seismic acquisition in shallow water areas and in transitional zone areas (i.e. areas that lie between dry land and open water). The operational constraints are primarily associated with the achievable data production rates, i.e. the rate at which seismic data can be acquired with a fixed amount of seismic equipment. Seismic equipment is expensive, and the per-day cost of using a piece of equipment is significant. Technical concerns are associated with the recoverable bandwidth and the amplitude of the source signal above the ambient noise conditions. Environmental concerns are the source's impact on marine life and the impact on the water bottom.
To create seismic images that are adequate for use in the exploration and production of oil and gas, the energy source must provide recoverable energy over a frequency range from six hertz to above one-hundred hertz and at amplitude levels significantly above the ambient noise environment. Additionally the energy source must be capable of operating at an acceptable production rate. Types of sources that have been used or proposed for shallow water seismic acquisition include small air gun arrays, auger air guns, explosive charges, accelerated weight drops, water column vibrators and bottom referenced vibrators or combinations of these sources.
Devices such as the accelerated weight drops and bottom referenced vibrators are capable of providing adequate energy in the desired bandwidth; but they also have the potential to significantly damage the water bottom. Placing a vibratory source in the water column or at the water surface eliminates the potential for damage to the water bottom; but reduces signal power per device, uses the water column to couple energy into the earth and significantly reduces the energy below twelve to fifteen hertz. A reduction in the frequency components below twelve to fifteen hertz dramatically reduces the potential to adequately image for deep seismic data. The reduction in the low frequency components can be compensated for by increasing the dwell time for the lower frequency components; but this technique can significantly increase the time spent for each source point with a corresponding increase in the cost and time duration of the seismic survey.
So bottom referenced vibrators show better potential for seismic data acquisition, but can be more damaging to the environment. In early U.S. Pat. No. 3,365,019, a vibrator system is housed in an earth coupling or foot member that engages the soft waterbed bottom with a cup-like, cylinder-shaped object after being lowered by a cable from a suitable vessel or vehicle. Vacuum creating means are included in the coupling member to assure adequate contact with the water-bottom surface. When removal or transfer to a different location is desired, the vacuum is replaced with an over-pressure from a suitable air pump for disengagement. It is clear that sub-surface penetration is required as are power sources for adjusting the pressure in the coupling element. U.S. Pat. No. 4,867,096 uses a cable suspended, pointed rod to penetrate water-bottom areas and to provide vibratory signals directly into that sub-surface. The vibratory source is affixed at the top of the rod to impart the vibration transmitted into the earth by the rod; the whole assembly is again lowered by cable from an attending vehicle or vessel suitable for use in marshlands or shallow water. Again, penetration can damage sensitive wetlands and power sources adequate to withdraw the rod can become burdensome economically and physically when trying to minimize overall system weight. Another rod or pipe method is provided in U.S. Pat. No. 5,031,718. Here the elongated coupling means extends from the vessel or vehicle, seats on the bottom and engages it. A driver means is used to created sound in the coupling, and can be on the vessel or vehicle, or can be located in the baseplate system. In soft bottoms often present in shallow waters or promising marshlands, significant penetration is likely with the recommended coupling, such as the suggested pipeline section. Similar problems as noted for U.S. Pat. No. 4,867,096 can be anticipated.
For shallow water seismic acquisition, an energy source is needed that provides a high-amplitude, broadband seismic signal, provides high data-production rates, and couples energy directly into the earth and minimizes damage to the water bottom.